DINOSAURS Flashcards
What group did dinosaurs evolve from?
The Archosauria
(a monophylectic group)
What are the synapomorphies of archosaurs?
- Presence of calcaneal tuber - bone projecting from ankle - flexor muscle is attached
- 2 skull opening - one in lower jaw, hypothesised that it was where jaw muscles attached.
- Second skull opening is between eye socket and nostrils - hypothesised: adaptation for reducing skull weight
- Presence of teeth in sockets - roots that are not fused to the jaw. Thecodont.
- Reduction in the 5th toe
- Secondary palate (allow them to breath whilst eating)
What are the 2 major clades within archosauria?
- Pseudosuchia
- Ornithodira
What were the Pseudosuchia?
- Reptiles & crocodilians
- High diversity during mid - late triassic
- Only extant group crocodiles
- Quadrupeds and some facultative bipeds
What were the Ornithodira?
- Pterosaurs, dinosaurs + primitive relatives
- Non-avian dinosaurs and birds
- Simple hinge-joint ankle
- Very long hindlimbs
- Longer s-shaped neck
Which was more prominant, Pseudosuchians or dinosaurs when dinos first appeared?
- Pseudosuchians
- Ruled by reptiles
What were the shared derived features of dinosaurs?
- The semi-erect posture (more upright)
- Limbs positioned directed below the shoulders
- Pubis bone - downwards and backwards
- Hip joint
- S-shaped neck
- Reduction in 4th and 5th digit on hand
- Reduction to 3 main toes on foot
What are the characteristics of the dinosauria hip?
- Open acetabulum (hip socket)
- Sacrum - fused vertebrae
What are the two major clades within dinosauria?
- Ornithischians
- Saurischians
What is the clade saurischians comprised of ?
- Sauropodomorphs
- Theropods (including birds)
How did two groups evolve based on the study of the hip joint ?
- Difference is in the pubis bone
- In Ornithischian the pubis is aligned and parallel with the ischium - facing backwards by tail
- In Saurischians pubis points forward and flares into keel
How is the hip positioned in the Ornithischians?
pubis is aligned and parallel with the ischium - facing backwards by tail
How is the hip positioned in the Saurischians?
pubis points forward and flares into keel
Which group are ‘Lizard Hip Dinosaurs’?
Saurichians
Which group have ‘Bird Hips’?
Ornithischians
Which hip dinosaur group did modern birds evolve from?
Lizard Hip Dinosaurs
Bird hipped dinosaurs did not include birds
How has dinosaur phylogeny been reworked?
- Evolutionary tree was shuffled about and a new phylogeny hypothesis was established including new clade Ornithocelidans. WIP
- This would mean birds evolved from bird-hip dinosaurs
Features of reproduction in dinosauria
- Study extant dinosaurs and infer behaviour of dinosaurs
- All extinct dinosaurs laid eggs (this is known)
- Hard-calcium carbonate shell - was widely accepted to be the only type of eggs that dinosaurs laid.
- Did they develop from soft egg shells? Leathery (like turtle eggs)
- Fossil bias - most are from cretaceous - were the earlier eggs soft and so unpreserved
- Sexual dimorphism - larger females?
- Male competition - all very hypothetical
Diversity of dinosaurs
- More is known about the extinction of the dinosaurs that how they diversified
- 700 - 800 groups
- Only around 5% excavated from triassic rocks
- Birds are the only group of dinosaurs that survived from the cretaceous period
What is the most accepted hypothesis for the extinction for non-avian dinosaurs?
Asteroid in the yucatan peninsula
Were dinosaurs declining before the extinction event?
- End of cretaceous was a changing time (environmental etc)
- Study suggested that dinosaurs were declining about 10 million years before the asteroid caused extinction
What drove the delcine in dinosaurs before the asteroid?
- Abiotic factors (climate, temperature, sea level etc)
- Biotic factors (diversity of ferns, evolution of angiosperms etc)
- Diversity dependence (interaction between carnivores and herbivores)
Two factors were at play that explain decrease
- Temperature - global cooling climate
- Density of herbivorous dinosaurs
Birds evloved in the triassic, diversified in the jurrasic and are the only group of dinosaurs to survive the end of the cretaceous period
